Project Summary/Abstract Metastatic recurrence in breast cancer patients accounts for over 90% of cancer-associated deaths, claiming ~40,000 lives in the United States every year. Metastatic disease is refractory to anti-cancer therapies that have utility against the primary tumor, which underscores our need for a better understanding of the biology regulating metastasis. Autophagy, an intracellular self-digestion process, has emerged as a potential therapeutic target in metastasis and several studies suggest that autophagy promotes the metastatic properties of tumor cells in vitro. In the clinical setting, anti-malarials (i.e., chloroquine) have been repurposed as cancer chemotherapies for solid tumors, including advanced metastatic tumors, as they inhibit late stages of the autophagic pathway; however, the precise impact of autophagy inhibitors on metastatic disease in vivo remains unclear. In contrast with previous studies in the field, including published work from my own lab, my preliminary data demonstrates a dual and highly stage-specific role for autophagy during metastasis. I have generated mice in which autophagy can be conditionally deleted in mammary tumor cells during tumorigenesis driven by the Polyoma Middle T antigen (PyMT). Using these powerful genetic models, I have found that intact autophagy promotes primary mammary tumor growth as well as the early stages of metastatic seeding, but it surprisingly restricts metastatic colonization by preventing the outgrowth of tumor cells that have already seeded the metastatic microenvironment. Motivated by this preliminary data, my specific aims will address two fundamental questions on the stage-specific roles of autophagy during metastasis: (1) What is the underlying mechanism of autophagy in preventing efficient metastasis colonization? (2) Is there utility in genetically and pharmacologically targeting autophagy for preventing metastatic recurrence? These studies will broaden our understanding of the requirements for metastasis and will inform therapeutic approaches for preventing metastasis that can be rapidly translated to the clinic. Moreover, since patients with invasive ductal carcinoma (IDC) of the breast frequently have disseminated tumor cells residing at foreign tissue sites at the time of diagnosis, my studies will delineate the utility of modulating autophagy as a therapeutic target to prevent these disseminated tumor cells from erupting into overt metastatic disease.